The invention relates to a device for parallax-free centering of an eyeglass lens blank and for applying markings and/or a block holder prior to mounting the eyeglass lens blank in an eyeglass lens edge grinding machine, in which a light divider located between the eye and the eyeglass lens blank superimposes upon the image of the eyeglass lens blank an image of a template or of an eyeglass frame opening and of a scale.
In devices of this type, such as the one described for example in DE 40 12 661 A1 by the same applicant, an eyeglass lens blank placed on a translucent stage which is illuminated from below is then viewed through a light divider. Due to the superimposition of an image of a template or of an eyeglass frame opening and of a scale the observer can determine whether the selected diameter for the eyeglass lens blank is suitable for the selected shape of the template and/or the eyeglass frame taking into account the decentration values for the wearer of the eyeglass frame, i.e. is neither too large nor too small. The eyeglass lens blank can be shifted laterally on the stage and adjusted, using the scale, to match the decentration values entered by the optician. Then a block in the form of a sucker or adhesive block is attached at the points on the eyeglass lens blank corresponding to the decentration values or an appropriate marking is applied. The image of the template or the eyeglass frame opening can be imaged by means of a liquid crystal display screen located in the optical path of the light divider. This liquid crystal display screen can also generate a representation of the scale.
To achieve parallax-free representation of the eyeglass lens blank, the superimposed image of a template or of an eyeglass frame opening and of a scale, the liquid crystal display screen and the upper surface of the eyeglass lens blank should be equidistant from the point at which the images are merged in the light divider. When using a liquid crystal display screen located in the viewer's line of sight, this means that the distance of the liquid crystal display screen from a semi-transparent surface of the prism should be equal to the sum of time distances of the eyeglass lens blank from a totally reflective surface of the prism and of this surface from the semi-transparent surface of the prism.
However, to be able to place the eyeglass lens blank on the stage and to be able to join the upper surface of the eyeglass lens blank with the block component it is desirable to design the stage and a depressor which holds the eyeglass lens blank on the stage so as to be upwardly and downwardly movable ensuring thereby, however, that centering is always effected at the prescribed distance from the light divider.
Accordingly, the object of the invention is to improve a device of the type mentioned at time outset in such a way that centering is always carried out at a pre-determinable distance from the light divider in spite of the upward and downward movability of the stage and the depressor.